An aircraft is an assembly of numerous interacting mechanical and electrical components that need to function properly for the aircraft to operate safely. An important step in the manufacture and maintenance of an airplane is the testing of the components in order to verify that they operate accurately and reliably. Almost all aircraft components are tested prior to installation in the aircraft. Most aircraft components should also be tested after installation to verify that they function properly in conjunction with the other components to which they are connected. This testing is necessary to verify that the aircraft, as an assembly of interacting components, will function properly when in flight.
Conventional testing of aircraft components relies on either flight testing of the aircraft or system testing when the aircraft is on the ground. Flight testing, as the name implies, involves actually flying the aircraft in order to ensure that its various components operate properly with respect to each other. Flight testing, while useful, has limitations. In flight testing it is difficult, if not impossible, to expose an aircraft to all conditions and events to which it might be exposed in order to observe how the components will respond. For example, environmental conditions such as sudden cross winds cannot be developed on command. Also, complex aircraft, such as modern commercial airliners, comprise a large number of components that are assembled into numerous systems. These components and systems must be subjected to numerous test procedures in order to ensure that they function properly. It is neither an efficient use of time nor resources to repeatedly flight test an aircraft solely to ascertain whether a specific assembly of components are performing satisfactorliy. Moreover, a number of test procedures involve verifying the proper operation of aircraft under potentially threatening conditions such as low altitude, high wind conditions. Testing the in-flight response of an aircraft to these conditions and events has the potential of jeopardizing the safety of both the aircraft and the flight crew.
The alternative to flight testing an aircraft is ground testing. This involves applying input signals to the components of a grounded aircraft and monitoring the response. Conventional ground testing equipment is typically only useful for testing the components forming one or two specific aircraft systems. For example, some ground test equipment is useful for testing an aircraft's radio navigation equipment. However, this equipment is of little value for testing the performance of other aircraft components in response to information from both the radio navigation equipment and the aircraft inertial reference navigation equipment. Moreover, many conventional ground test systems can only be used to test some of the components that form only part of a specific aircraft system. For example, some ground test equipment is only useful for testing aircraft components that respond to the signals produced by the pressure-sensitive altimeter; this equipment cannot be used to test the actual pressure-sensitive elements of the altimeter. Therefore, this equipment is not particularly useful for testing an altimeter system so as to observe how the whole system, and the components to which it is connected, function as an integrated system. Consequently, ground testing is of limited value for observing how the components and systems that form an aircraft will function together when the aircraft is in flight.